There are a large number of pocket universes and sub-dimensions that are parallel to this universe. Most are limited in size and scope, and only accessible to (or even known by) small groups with access to special technology or unusual abilities.
(While the Negative Zone is technically another dimension, its anti-matter nature is unique and inimical in ways that most other dimensions are not.)
A huge number of alternate/alternative realities exist, in which history or reality unfolded in different ways. This universe may in fact be part of a larger, possibly infinite multiverse of timelines and realities. Most of the traffic to these realities seems to be organised by humans.
Home to the ultradimensional ‘gods’ called Asgardians, who are all currently believed to be dead.
The Darkforce Dimension
A pocket dimension composed entirely of semi-solid, controllable darkness-energy (‘darkforce’).
A miniature dimension accessible only through vibrational compression (shrinking). Home of the Micronauts.
Home of Merlyn and the Captain Britain Corps, and location of Camelot and the Green Chapel.
Who the hell knows.